The weight of the nuclear reactor fuel bundle or assembly used in certain nuclear reactors, such as commercial light water reactors, consists primarily of an upper end fitting, the fuel rods, a lower end fitting, and the intermediate grid cages.
When handling the fuel bundle, a fixture such as a refueling mast or grapple is connected to the upper end fitting. When the fuel bundle is lifted, most of the weight of the fuel bundle is carried by the fuel rod guide tubes that are connected to the upper end fitting and which descend through the intermediate grid cages. Consequently, the structural integrity of the guide tubes must be intact in order to safely move the fuel bundle.
In some cases, the structural integrity of the guide tubes is not intact, or the guide tube may at least be of questionable structural integrity. Guide tubes have experienced problems with the formation of defects that may be caused by, for example, stress corrosion cracking that may be induced by the service environment within a nuclear reactor or spent fuel pool. If such cracks or other flaws are present or suspected to be present in the guide tubes, the structural integrity of the guide tube must be improved in order to render the fuel bundle safe for handling.
FIG. 1 depicts apparatus that has been used in attempts to repair the structural integrity of a guide tube (2) shown attached to a portion of a fuel bundle (3). The apparatus includes a sleeve (4) and a threaded rod (6). The sleeve includes a clamping portion (8), a threaded portion (10), and a flared surface (12). The threaded rod has a flared surface (14) and a threaded portion (16) that screws into the threaded portion (10) of the sleeve.
By turning the bolt head (18) in the appropriate direction, the threaded rod is moved axially relative to the sleeve so as to pull the rod""s flared surface (14) into the flared surface (12) on the sleeve. Further turning of the bolt head will cause the clamping portion (8) to expand from a relaxed position and into load bearing contact with the inside surface of the guide tube. The sleeve can have one or more stress relief slots (not shown), extending over its longitudinal length to facilitate the expansion of the sleeve. A separate locking means (not shown) is used to prevent the pin from rotating and thus maintain the pin the in sleeve.
Given the need for both safety and speed when moving nuclear reactor fuel bundles, it desirable to have an improved means for assuring the structural integrity of a guide tube.
The anchor assembly apparatus of the present invention is used to improve the structural integrity of a guide tube and comprises an elongated sleeve that can be installed into the guide tube through a receiving end at the upper end fitting and an elongated pin. The elongated pin is inserted into the sleeve at the upper end fitting, causing a portion of the sleeve to radially expand into and contact an inside surface of the guide tube, thus imparting a radial force into the guide tube. The expansion is such that the radial forces developed between the sleeve, pin, and guide tube, lock the sleeve and pin into place in the guide tubes. With these members thus installed, the sleeve can carry the loads that would otherwise be carried by the guide tube, or portions of the tube that contain a structural flaw.
The sleeve has a proximate end, and a distal end that is adapted for insertion into the guide tube. The pin has a near end, as well as a far end adapted for insertion into the sleeve at the sleeve""s proximate end. The pin includes an external surface that applies a radial force to an inner surface of the sleeve when the pin is inserted into the sleeve. The force is sufficient to cause a locking surface on the sleeve to expand radially outwards. Thus, when the sleeve is installed into the guide tube, and the pin is installed into the sleeve, the locking surface bears against the guide tube to retain the sleeve in the guide tube, and the external surface bears against the inner surface to retain the pin in the sleeve.
According to another aspect of the invention, the proximate surface has a load bearing surface adapted to engage a portion of the fuel bundle in a load bearing relationship.
According to another aspect of the invention, at least part of the internal surface is tapered, and at least part of the external surface is tapered complementary to the internal surface.
According to another aspect of the invention, the internal surface has a taper ratio of about 0.125 inches for every 12 inches.
According to another aspect of the invention, the internal surface extends from at least about the distal end towards the proximate end.
According to another aspect of the invention, the sleeve has at least one stress relief slot extending from at least about the distal end towards the proximate end.
According to another aspect of the invention, the locking surface comprises one or more rings.
According to another aspect of the invention, apparatus is provided for improving the structural integrity of a guide tube in a nuclear fuel bundle wherein, the guide tube has a structural flaw positioned at a flaw distance from a guide tube receiving end. The apparatus includes an elongated sleeve with a proximate end, a distal end adapted for insertion into the guide tube receiving end, and a locking surface. The locking surface is positioned from the proximate end a grip length that is longer than the flaw distance. The apparatus also includes an elongated pin having a near end, and a far end adapted for insertion into the sleeve at said proximate end. The elongated pin has an external surface that applies a radial force to an inner surface of the elongated sleeve when the elongated pin is installed in the elongated sleeve, and the radial force is sufficient to cause the locking surface to expand radially outward. Consequently, when the elongated sleeve is installed in the guide tube, and the elongated pin is installed in the sleeve, the locking surface bears against the guide tube to retain the elongated sleeve in the guide tube, and the flaw is positioned between the proximate end and the locking surface.
According to another aspect of the invention, the proximate end is adapted to engage a portion of the fuel bundle in a load bearing relationship.
According to another aspect of the invention, the internal surface and the external surface are self-locking.
According to another aspect of the invention, in a nuclear fuel bundle having one or more guide tubes, wherein the guide tubes function as load carrying members when the fuel bundle is moved, a method is provided for improving the structural integrity of a guide tube. A sleeve is provided having a load surface adjacent a proximate end and a locking surface adjacent a distal end. The distal end of the sleeve is inserted into a receiving end of the guide tube, and the sleeve is moved into the guide tube until the load surface engages a portion of the fuel bundle, and the locking surface is moved until the locking surface engages the guide tube in a load bearing relationship.
According to another aspect of the invention, a pin is inserted into the sleeve through the sleeve""s proximate end, until an external surface on the pin contacts an internal surface on the sleeve adjacent the locking surface, and a force is applied to the pin to move the pin a further distance into the sleeve to cause the locking surface to radially expand into the guide tube.
According to another aspect of the invention, the location of a flaw in a guide tube is determined, and a sleeve is provided wherein the flaw will be positioned between the load surface and the locking surface when the sleeve inserted into the guide tube.
According to another aspect of the invention, the force applied to the pin is measured.
According to another aspect of the invention, the force applied to the pin is compared with a predetermined acceptable force range.
According to another aspect of the invention, the further distance that the pin moves is measured.
According to another aspect of the invention, the further distance that the pin moves is compared with a predetermined acceptable length.